The present invention relates generally to imaging and, more specifically, to a low cost high efficiency signal interrogation technique for multi-channel optical coherence tomography.
When lights reflected from samples interfere with a reference beam, the frequencies of the interfering signals reveal the depth where the light is reflected. This technique has been used in imaging, known as Optical Coherence Tomography (OCT). OCT is an optical signal acquisition and processing method allowing extremely high-quality, micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue) to be obtained. In contrast to other optical methods, OCT, an interferometric technique typically employing near-infrared lights, is able to penetrate significantly deeper into the scattering medium, for example about three times deeper than confocal microscopy.
The first generation OCT is a time domain technique that uses a wideband light source and a time delay scanner. Only when the optical paths of the reflection lights and the reference beam are matched, they can interfere and be detected. The significant drawback of this technique is a low imaging speed, which is limited by the speed of the delay line scanner.
In order to improve the imaging speed, a second type of OCT called Spectral Domain OCT (SD-OCT) has been developed. Similar to the time domain OCT, this technique also uses a broadband light source. Instead of the time delay scanner, a transmission grating and a CCD array are used to interrogate the interfering signals. Since the speed for CCD array scanning can be very high, this technique can be used for very high speed 3D imaging. The disadvantages, however, are the heightened costs, the limited imaging depth and resolution.
Fourier domain OCT (FD-OCT) that uses wavelength-swept fiber laser sources is the third type of OCT. The coherence length resulting from the narrow instantaneous laser linewidth enables imaging up to 4 mm depth in tissue. The wavelength sweeping rate has reached 100 kHz, which is fast enough for 3D imaging in many applications. Of the three typical OCT techniques described above, FD-OCT that uses wavelength-swept light sources is the most suitable for commercial purposes in biomedical imaging; this technique is cost effective, and has a faster imaging rate as well as improved resolution and sensitivity.
When a wavelength-swept light source and a fiber probe are used in OCT imaging, a 1D depth image, or A-scan, is obtained when the laser source makes a complete scan. When the fiber probe is scanned across an object, a series of A-scans produce a 2D cross-section image, or B-scan. When a series of 2D section imaging are accumulated, a 3D image is obtained.
For some applications, however, the process of scanning may be inconvenient or not economical, such as in catheter imaging. Probe arrays could be used instead of scanning to form a multi-channel OCT. The prototyped 5-channel OCT has been developed by Thorlabs, using an optical switch, with five photo detectors and fiber circulators, to measure five channel signals, respectively, as shown in FIG. 1. For a 16-channel OCT, the cost for the signal interrogation would be over $30,000 at the time of drafting this patent application. Such a high cost renders it unsuitable for commercial purposes. Apart from the high cost, the cross-talking, and the bulk size, the most critical drawback is that this interrogation technique is not suitable for a balanced detection to remove the strong low frequency signal background, which seriously reduces the measurement sensitivity.
Multi-channel OCT can be used to measure multiple distances that could be used to investigate in real time strain, force, temperature, and the like. When a force is applied to a spring or an elastic material, three channels OCT can monitor in real time three distances that could be used to measure the force directions and its amplitude. If the distance changes as a result of a thermal expansion material, one channel OCT could be used to measure temperature in real time.